1. Field of the Invention
The present invention relates to a light emitting diode or LED used as a backlight for a liquid crystal display for performing a color display, illumination of a flush for photographing a still or moving picture, the other usual light emitting source for illumination, more specifically to an improvement in a white light emitting diode device (hereinafter, referred to as white LED) capable of emitting white light or light close to the white light.
2. Description of Related Art
Because an LED chip (hereinafter, referred to as LED element) is a semi-conductor element, it has been known that the LED element has a longer operating life and good driving characteristic, is compact, has effective light-emission, and bright light emitting color. Therefore, an LED device (hereinafter, merely referred to as LED) in which the LED element as the semiconductor chip is mounted on a substrate has been used widely as a compact illuminating source.
Recently, from the fact that high efficient LED element for emitting three original color lights of red (R), green (G) and blue (B), respectively has been developed, a multi-color mixing type LED comprising LED elements of R, G and B has been known as shown in a patent document 1, Japanese Patent Laid-Open H. 7-15044 (FIG. 1), in order to emit white light.
However, in the LED comprising a combination of the LED elements, because each LED element, that is to say, each of the R-LED element, the G-LED element and the B-LED element has an excellent monochromatic peak emission wavelength, if it is used for a light emitting source of white color system, there is a problem that the color range of the LED is small and the color of emitted light from the LED is unnatural; in other words, the LED has a poor color rendering property. Herein, “color rendering property” is the property of a light source concerning how an object exhibits its color when the object is illuminated by the light source. If a light from a device has an excellent color rendering property, that means the property of the light is very similar to the property of natural light. Considering the importance of the color rendering properties of illumination devices, the CIE (Commision Internationale de l'Eclairage) has determined an evaluation method for the color rendering property in 1964. According to this method, a series of reference light sources are determined where the reference light sources are selected depending on the color temperature of the light source to be evaluated. The color rendering index Ra is determined from the difference in the color of a predetermined test color between the case when it is illuminated by the reference light sources and in the case when illuminated by the light source to be evaluated. Color rendering index Ra takes a value between 0 and 100.
For example, in case of the LED comprising the combination of the LED elements of R, G and B as described in the patent document 1, an emission spectrum is as shown by S1 in FIG. 7A, a valley of spectrum intensity exists widely between R and G, G and B. In particular, a range of the valley which has 550 nm to 610 nm between R and G is also wide in width, and the decline of the spectrum intensity thereof is drastic.
That is to say, the emission spectrum of the LED is vastly different from a spectrum characteristic of natural light. Therefore, the color rendering property of the LED is poor, and Ra becomes about 12 (in case of the natural light, Ra=100). Meanwhile, when the color rendering property of the LED is poor, it is unsuitable to use the LED as an illumination source in a read-out device such as a scanner device and a photocopier for detecting reflected light on an object or other devices.
Accordingly, to improve the problem, a white LED in which an emission color of light from an LED element is color-converted by a fluorescent material has been developed as shown in Japanese Patent No. 2927279 (FIGS. 1 and 3) (patent document 2).
The structure of the white LED is shown in FIGS. 8A and 8B by modifying it into a structure of a surface-mounted LED for the sake of convenience. In FIGS. 8A and 8B, 110 is a white LED. The white LED 110 is formed by fixing a blue LED element 101 of an INGaN system on an insulative substrate 102 provided with electrode patterns 103 and 104 for connecting, connecting one electrode 101a or p-layer electrode of the blue LED element 101 with the electrode pattern 103 by a wire 106, after connecting the other electrode 101b or n-layer electrode with the electrode pattern 104 by a wire 106, sealing the LED element and the electrodes by a sealing member 107 comprising a resin in which a fluorescent material 108 of an yttrium, aluminum, garnet (YAG)-fluorescent material or the like is dispersed. With such structure, a portion of emitted light sb from the blue LED element 101 having a peak wavelength in the vicinity of 460 nm, for example, is absorbed in the above-mentioned fluorescent material 108 and is converted into light of yellow-green color sy whose peak wavelength is around 560 nm. As a result, an emission spectrum of the white LED 110 includes an emission from the blue LED element 101 having the peak wavelength of 460 nm and an emission from the fluorescent material 108 having the peak wavelength of 560 nm, as shown at S2 in FIG. 7B. As is clear from the above, because the white LED 110 emits in most of the range of visible light, it has an excellent color rendering property and an average color rendering index Ra exceeding 80. Consequently, the problem of the poor color rendering property as in the multi color mixing type LED in the patent document 1 can be partly solved and an improved LED is offered.
However, there is still a problem in the conventional white LED including the blue LED element and the fluorescent material of the YAG or the like as follows.
As is clear from the emission spectrum as shown at S2 in FIG. 7B, in the range particularly exceeding the wavelength of 650 nm of a red range, the intensity of the spectrum declines considerably compared with that of an emission spectrum in the other range of wavelength (visible light). Therefore, this results in lack of repeatability in the red range. For example, if an illuminated object having an object color in the red range is illuminated, the red component of the reflected or transmitted light declines greatly, accordingly the white LED lacks the repeatability of the object color, compared to natural light.
Therefore, for resolving the problem of the above-mentioned conventional fluorescent material-color mixing type white LED, a fluorescent material-color mixing type white LED with red complementary effect, in which the fluorescent material is combined with the blue LED element and a red LED element is further added, has been known as disclosed in Japanese Patent Laid-Open 2002-57376 (patent document 3).
The fluorescent material-color mixing type white LED with red complementary effect emits red and white lights simultaneously by adding the red LED element to the conventional fluorescent material-color mixing type white LED as shown at numeral 110 in FIGS. 8A and 8B. FIG. 9 illustrates an emission spectrum S3 of the conventional fluorescent material-color mixing type white LED with red complementary effect. In FIG. 9, there exist a peak p1 with great intensity in the vicinity of 450 nm which is a component of blue light, a peak p2 in the vicinity of 560 nm which is a component of yellow light, and a large peak p3 in the vicinity of 650 nm which is a component of red light emitted from the red LED element. In other words, in the fluorescent material-color mixing type white LED with red complementary effect, the spectrum, particularly, in the red range of visible light is reinforced, the repeatability in the red range is improved.
In the fluorescent material-color mixing type white LED with red complementary effect as shown in the patent document 3, the color rendering property and the repeatability in the red range are considerably improved in comparison with the R-G-B mixing type white LED (in patent document 1) and the fluorescent material-color mixing type white LED (in the patent document 2).
However, the color rendering property is not sufficient when using the white LED for a backlight of a full color display as a white light source, in particular, an emission component in the vicinity of 500 nm in a green area is insufficient as shown in the spectrum S3 in FIG. 9.
Moreover, if the white LED is used as illumination light in place of a fluorescent bulb, there is a problem that it becomes unnatural illumination that red glares, and it is not possible to substitute it for the fluorescent bulb as natural illuminating light.
This point will be described using a chromaticity diagram shown in FIG. 10.
In FIG. 10, chromaticity points of red emission of the red LED element, yellow emission of the fluorescent material and blue emission of the blue LED element (typical one, for example, in the range of 450 nm to 470 nm), in the fluorescent material-color mixing type white LED with red complementary effect are represented at cr, cy and cb, respectively. These chromaticity points are regarded to be located adjacent to a locus ST of monochromatic light. Now, when the red light is not energized and only the blue LED element is energized, the chromaticity of the white LED follows on a straight line L combining the points, cy and cb according to a percentage of the blue emission and the yellow emission (and fluorescent light), while the straight line L passes a chromaticity point c0 of white color or the vicinity thereof. Here, even if the white LED is intended to target white color, when the red LED element is energized to increase the repeatability of the red range, all the chromaticity points are moved toward the point cr as shown by arrow F, x coordinate of the chromaticity increases so as to be recognized in a state tinged with red.
Furthermore, otherwise, there is a problem that the red color light is recognized as a point like state, as viewed from above because a mixture of the red color light and the other color light is insufficient and is not achieved evenly.